1. Field of the Invention
This invention relates to a misfire-detecting system for internal combustion engines, and more particularly to a misfire-detecting system of this kind which has a function of detecting an abnormality thereof.
2. Prior Art
FIG. 1 shows the arrangement of an ignition circuit and a sparking voltage-detecting circuit incorporated in a misfire-detecting system proposed, for example, by U.S. Pat. No. 5,212,947. In the figure, illustration of a distributor is omitted.
As shown in the figure, a terminal T2 is connected to a base of a transistor 4 which has its emitter grounded. The terminal T2 is supplied with an ignition command signal from an electronic control unit (hereinafter referred to as "the ECU"), referred to hereinafter. An ignition coil 1 is comprised of a primary coil 2 and a secondary coil 3 which have ends thereof connected to a terminal T1 which is supplied with supply voltage VB. The other end of the primary coil 2 is connected to a collector of the transistor 4, while the other end of the secondary coil 3 is connected to a anode of a diode (reverse current-checking means) 5. Further, the cathode of the diode 5 is connected to a positive electrode of a spark plug 6, a negative electrode of which is grounded.
A sparking voltage sensor 7 is provided at an intermediate portion of a connecting line 13 which connects between the diode 5 and the spark plug 6. The sensor 7 is electrostatically coupled to the connecting line 13, and forms together therewith a capacitance of several pF's. An output terminal of the voltage sensor 7 is connected via a connecting line 14 to a non-inverting input terminal of an operational amplifier 12. Connected in parallel between the line 14 and the ground are a capacitor 8, a resistance 9 and a diode 10, and the diode 10 has its anode grounded. A diode 11 is connected between the connecting line 14 and a supply voltage-feeding line VBS (which is supplied with stabilized supply voltage) with its cathode being connected to the supply voltage-feeding line VBS. An output terminal of the operational amplifier 12 is connected to an inverting input terminal thereof and a terminal T3. Symbol C.sub.0 indicates a floating capacitance present in the vicinity of plug electrodes.
According to the sparking voltage-detecting circuit constructed as above, voltage between the spark plug electrodes is divided by the capacitance of the voltage sensor 7 and the capacitance of the capacitor 8, and then input to the operational amplifier 12. The operational amplifier 12 acts as an impedance converter, which outputs the input voltage as it is. The diodes 10 and 11 act to control the input voltage to the operational amplifier 12 to a range of 0 to the supply voltage VBS.
When an ignition command signal is input to the terminal T2, discharge current flows through the diode 5 and then between the electrodes of the spark plug 6, whereas current (reverse current) flowing from the spark plug 6 to the secondary coil 3 is checked by the diode 5. Consequently, the sparking voltage detected upon misfiring is maintained at a high value for a prolonged time period (as indicated by the broken line B' of FIG. 8B), whereby occurrence of a misfire can be detected with accuracy.
However, when the diode 5 as the reverse current-checking means becomes faulty (short circuit), reverse current is not checked by the diode, and hence electric charge stored in the floating capacitance C.sub.0 in the vicinity of the sparking plug electrodes is discharged as the voltage on the side of the secondary coil 3 lowers, so that the sparking voltage quickly decreases even when a misfire occurs, resulting in degraded accuracy of misfire detection.
Besides, even when the diode 5 functions normally, if the spark plug 6 is in a so-called smoldering state, i.e. a state where fuel adheres to the plug electrodes such that no spark ignition takes place, the insulation resistance between the electrodes of the spark plug lowers, so that discharge takes place between the electrodes of the spark plug even upon misfiring, similarly to normal combustion. Therefore, it has been difficult to discriminate a failure of the diode 5 from smoldering of the spark plug, etc. by monitoring the waveform of the sparking voltage, according to the conventional system.
Smoldering of the spark plug is a temporary phenomenon, which spontaneously disappears with the lapse of time, i.e. with operation of the engine. However, a failure of the diode 5 does not automatically heal in most cases, and therefore it is necessary to determine that the misfire-detecting system suffers from a failure when the diode 5 is faulty, and warn the driver or an occupant of the failure to take an appropriate failsafe action. That is, it is necessary to discriminate a failure of the diode 5 from smoldering of the spark plug, etc.